The present invention relates to a method and apparatus for electrochemically-assisted or -augmented mechanical planarization, i.e., electrochemical-mechanical planarization (xe2x80x9cEMPxe2x80x9d), which method and apparatus enjoy particular utility in the manufacture of semiconductor integrated circuit devices.
Chemical-mechanical polishing (CMP) techniques and apparatus therefor have been developed for providing smooth topographies, particularly on the surfaces of layers deposited on semiconductor substrates during integrated circuit manufacture. In such instances, rough topography results when metal conductor lines are formed over a substrate containing device circuitry, e.g., inter-level metallization patterns comprising a plurality of electrically conductive lines which may, inter alia, serve to interconnect discrete devices formed within the substrate. The metal conductor lines are insulated from each other and from vertically adjacent interconnection levels by thin layers of dielectric insulation material, and openings formed through the insulating layers provide electrical interconnection and access between successive such interconnection levels. In fabricating such type devices including multiple interconnection and insulative layers, it is desirable that the metallic and insulative layers have a smooth topography, inasmuch as it is very difficult to photolithographically image and pattern layers applied to rough surfaces. CMP can also be employed for removing different layers of material from the surface of a semiconductor substrate, as for example, following via hole formation in an insulating layer, when a metallization layer is deposited and then CMP is used to form planar metal via plugs embedded in the insulating layer.
Briefly, CMP processes utilized in semiconductor device manufacture involve mounting a thin flat workpiece, e.g., a semiconductor wafer substrate, on a carrier or polishing head, with the surface to be polished being exposed. The substrate surface is then urged against a wetted polishing surface, i.e., a rotating polishing pad, under controlled mechanical pressure, chemical, and temperature conditions. In addition, the carrier head may rotate to provide additional motion between the substrate and polishing pad surfaces. A polishing slurry containing a polishing agent, such as alumina (Al2O3) or silica (SiO2) finely-dimensioned particles is used as the abrasive material. Additionally, the polishing slurry contains a number of chemicals, including pH adjusting and stabilizing agents, as well as chemical oxidizing agents for chemically removing (i.e., etching) various components of the surface being planarized. The combination of mechanical and chemical removal of surface material during the polishing process results in superior planarization of the polished surface, vis-à-vis other planarization techniques.
Slurries used for CMP can be divided into three categories, depending upon their intended use: silicon (Si) polish slurries, oxide polish slurries, and metal polish slurries. Si polish slurries are designed to polish and planarize bare Si wafers and are typically composed of very small (i.e., about 20-200 nm diameter) abrasive particles, e.g., of silica (SiO2), alumina (Al2O3), or ceria (CeO2), suspended in a water-based liquid at a somewhat basic pH provided by a pH adjusting agent, typically a hydroxide-type base. Oxide polish slurries are designed to polish and planarize a dielectric layer on a wafer, typically a layer of silicon dioxide (SiO2), and are similarly composed of very small abrasive particles (i.e., about 20-1000 nm diameter) of, e.g., SiO2, Al2O3, or CeO2, suspended in a water-based liquid at a high (i.e., basic) pH.
Metals polish slurries are designed to polish and planarize conductive layers on semiconductor wafer substrates. The conductive layers are typically deposited on a dielectric layer and typically comprise metals such as tungsten (W), titanium (Ti), aluminum (Al), copper (Cu), alloys thereof, semiconductors such as doped silicon (Si), doped polysilicon, and refractory metal silicides. The dielectric layer typically contains openings (xe2x80x9cviasxe2x80x9d) that are filled with the conductive material to provide a path through the dielectric layer to previously deposited layers. After the conductive layer is polished, only the conductive material filling the vias remains in the dielectric layer.
Metal polish slurries utilized for such CMP of vias typically include very small particles (i.e., about 20-1000 nm diameter) of the above-mentioned abrasive materials, suspended in a water-based liquid. In contrast to the Si and oxide-type polishing slurries, the pH may be acidic (i.e.,  less than 5) or neutral and is obtained and controlled by addition of acid(s) or salt(s) thereof. In addition to the organic acid(s) or salt(s), metals polishing slurries include one or more oxidizing agents for assisting in metal dissolution and removal, typically selected from hydrogen peroxide, potassium ferricyanide, ferric nitrate, or combinations thereof.
However, the combination of acidic or neutral pH and presence of oxidizing agent(s), hereafter xe2x80x9coxidizer(s)xe2x80x9d, in CMP metals polishing slurries can result in several disadvantages, drawbacks, and difficulties, including, inter alia:
(a) the oxidizer can continue to etch the electrically conductive material, e.g., metal, during xe2x80x9cstaticxe2x80x9d periods, i.e., periods when mechanical polishing is not being performed but the substrate surface remains in contact with the polishing slurry containing at least one oxidizer, e.g., upon completion of CMP but prior to removal of the substrate surface from contact with the slurry. As a consequence, unwanted static etching of the metallic features of the polished surface can occur, disadvantageously resulting in formation of depressions therein, referred to as xe2x80x9cdishingxe2x80x9d, which phenomenon remains a significant problem in metal CMP processes;
(b) the amount of oxidizer present in the metals polish slurries is not constant during the interval necessary for completion of the CMP processing, but rather varies during the course of CMP as a result of consumption thereof during the metal oxidation process. As a consequence, the concentration of oxidizer in the slurry, hence the rate of metal oxidation, is not controlled throughout processing, unless continuous, reliable detection/concentration measurement and replenishment means are provided, which means undesirably add to the cost of CMP processing;
(c) in some instances, the presence of oxidizer in the metals polishing slurry is particularly undesirable during a specific portion of the CMP processing. For example, the presence of oxidizer in the slurry during the later stage(s) of polishing frequently results in the above-mentioned problem of xe2x80x9cdishingxe2x80x9d, i.e., a height differential between a dielectric oxide layer and metallization features within an array of metallization features, as well as undesirable corrosion and xe2x80x9cerosionxe2x80x9d, i.e., a height differential between a dielectric oxide layer in an open field region and in an array of metallization features; and
(d) the presence of oxidizer(s) and spent oxidizer(s), e.g., peroxide, Fe ions, etc., in spent (i.e., waste) abrasive slurry adds to the complexity, problems, and expense associated with handling and disposal of the waste slurry in an environmentally acceptable manner.
U.S. Pat. No. 4,839,005 discloses a method and apparatus for providing mirror-smooth finishes to aluminum surfaces by applying a constant anodic potential to the surface via a passivation-type electrolyte solution, while simultaneously performing mechanical polishing thereof with an abrasive slurry or cloth. While such electrolytically-assisted polishing may dispense with the requirement for a chemical oxidizer in the polishing slurry or abrasive cloth, the application of a constant anodic potential renders the disclosed method/apparatus unsatisfactory for use in the planarization of workpieces comprising semiconductor wafers with surfaces having electrically conductive wirings, etc., in that undesirable dishing would still occur during the late stage(s) of planarization due to excessive, electrochemically promoted anodic metal dissolution, as in the conventional methodology employing chemical oxidizing agent(s).
Accordingly, there exists a need for a simplified and reliable method and apparatus for performing planarization processing, particularly of semiconductor wafer substrates comprising surfaces including metallization patterns on or within a dielectric material layer, which method and apparatus are free of the disadvantages and drawbacks associated with the conventional CMP methodologies, and are fully compatible with the economic and product throughput requirements of automated semiconductor manufacture processing.
The present invention addresses and solves the above-described problems attendant upon the manufacture of integrated circuit semiconductor and other electrical and electronic devices according to conventional CMP methodology utilizing abrasive slurries containing chemical oxidizer agent(s), and is fully compatible with all other mechanical aspects of CMP-type planarization processing.
An aspect of the present invention is an improved method of planarizing workpiece surfaces comprising an electrically conductive material.
Another aspect of the present invention is an improved method of planarizing a workpiece surface by means of an electrochemical-mechanical planarization (EMP) process and apparatus utilizing an abrasive slurry free of chemical oxidizing agent(s).
Yet another aspect of the present invention is an improved method of controllably planarizing a workpiece surface.
Still another aspect of the present invention is an improved method of controllably planarizing a semiconductor substrate surface comprising a pattern of electrical conductors.
A still further aspect of the present invention is improved apparatus for controllably performing EMP of at least one surface of at least one workpiece.
A yet another aspect of the present invention is improved apparatus for performing EMP of a workpiece surface with an oxidizer-free abrasive slurry.
Additional aspects and other features of the present invention will be set forth in the description which follows and in part will become apparent to those having ordinary skill in the art upon examination of the following or may be learned from the practice of the present invention. The advantages of the present invention may be realized and obtained as particularly pointed out in the appended claims.
According to one aspect of the present invention, the foregoing and other advantages are achieved in part by a method of planarizing a surface of a workpiece by electrochemical-mechanical planarization (EMP), which method comprises the steps of:
(a) providing a chemical-mechanical polishing (CMP) apparatus with at least one workpiece having at least one surface to be planarized by EMP;
(b) supplying the CMP apparatus with an electrolytically conductive fluid free of chemical oxidizing agent(s); and
(c) planarizing the at least one workpiece surface by EMP utilizing the CMP apparatus, the planarizing by EMP including applying a time-varying electrochemical potential to the at least one workpiece surface.
In embodiments according to the present invention, step (c) comprises performing EMP by controllably applying a time-varying anodic potential to the at least one workpiece surface. In particular embodiments according to the present invention, step (c) comprises applying a first, higher anodic potential during an initial stage of the EMP and applying a second, lower anodic potential at or during a later stage of the EMP; or step (c) comprises reducing the first, higher anodic potential to the second, lower anodic potential during an intermediate stage of the EMP, e.g., continuously reducing the anodic potential during the intermediate stage; or step (c) comprises rapidly reducing the anodic potential from the first, higher potential to the second, lower potential after a predetermined interval at the first, higher potential.
According to further embodiments of the present invention, step (c) comprises applying the time-varying electrochemical potential from a controllably variable DC power supply, e.g., an electronic potentiostat; and a further step (d) comprises monitoring the extent of the EMP of the at least one workpiece surface in order to determine the end-point thereof. According to particular embodiments of the present invention, step (d) comprises coulometrically monitoring the extent of EMP, or step (d) comprises monitoring a signal from a sensor utilized for measuring a physical property (e.g., electrical resistance or conductance)) or optical property (e.g., reflectance) of the at least one workpiece surface.
According to yet further embodiments of the present invention, step (a) comprises providing a semiconductor wafer substrate as the workpiece, the semiconductor wafer substrate comprising the at least one workpiece surface and including a pattern of electrical conductors formed on or within a layer of a dielectric material; and step (b) comprises supplying the CMP apparatus with an oxidizer-free, electrolytically conductive, abrasive slurry comprising finely-dimensioned abrasive particles and at least one pH adjusting agent.
According to still further embodiments according to the present invention, step (a) comprises providing a CMP apparatus having a non-abrasive polishing pad and step (b) comprises supplying the CMP apparatus with an oxidizer-free, electrolytically conductive fluid comprising an abrasive slurry comprising finely-dimensioned abrasive particles; or step (a) comprises providing a CMP apparatus having an abrasive polishing pad and step (b) comprises supplying the CMP apparatus with an oxidizer-free, electrolytically conductive fluid comprising a non-abrasive liquid; or step (a) comprises providing a CMP apparatus having a non-abrasive polishing pad and step (b) comprises supplying the CMP apparatus with an oxidizer-free, electrolytically conductive fluid comprising a non-abrasive liquid.
According to another aspect of the present invention, apparatus for performing electrochemical-mechanical planarization (EMP) of at least one surface of at least one workpiece comprise:
(a) a device adapted for performing chemical-mechanical polishing (CMP) of the at least one workpiece surface; and
(b) a power supply connected to the CMP-adapted device for providing a controllable, time-varying DC electrochemical potential to the at least one workpiece surface for effecting EMP thereof.
In embodiments according to the present invention, the device (a) adapted for performing CMP of at least one workpiece surface comprises an abrasive polishing pad or a non-abrasive polishing pad; power supply (b) comprises an electronic potentiostat adapted for applying a time-varying, anodic electrochemical potential to the at least one workpiece surface; and the apparatus further comprises a device (c) for monitoring the extent of EMP of the at least one workpiece surface for determining the end-point thereof. According to particular embodiments of the present invention, the monitoring device (c) comprises a coulometer or a sensor for providing a signal indicative of a physical (e.g., electrical) or optical property of the at least one workpiece surface.
Additional advantages of the present invention will become readily apparent to those skilled in the art from the following detailed description, wherein only the preferred embodiment of the present invention is shown and described, simply by way of illustration of the best mode contemplated for carrying out the method of the present invention. As will be described, the present invention is capable of other and different embodiments, and its several details are capable of modification in various obvious respects, all without departing from the present invention. Accordingly, the drawing and description are to be regarded as illustrative in nature, and not as limitative.